According to the TV station, the accident happened on January 28 during the filming for "3B junior no Hoshikuzu Shouji", a variety show in which the members challenge themselves to various tasks to improve their individual skills.
On this day, the members were playing a game using a commercially available party product which contained helium gas, and "A" suddenly collapsed. After she lost consciousness, she was taken to a hospital by ambulance, and received emergency care.
"A" is currently under treatment by a medical specialist at the different hospital, and she was diagnosed with air embolism in brain which is when air or as bubbles get in the blood vessel and blocks blood flow. Though she is showing signs of recovery, she has not gained complete consciousness.
The product they used said "for adult" on it, and there was a list of notes, however, the staff members failed to notice that.
TV Asahi says that there was possibly a problem with the safety management, and apologized to the member, her family, and the related party. They also released a comment that they would investigate the cause thoroughly and plan on prevention of recurrence, and make make an effort for better safety management.